1. Field of the Invention
The invention relates to an electric power steering device that executes a control of restraining the overheating of an electric motor provided for steering assist.
2. Description of the Related Art
An electric power steering device mounted in a vehicle includes an electric motor that is driven according to the steering torque applied to a steering member, and executes steering assist by transmitting the drive force generated by the motor to a steering mechanism of the vehicle. In such an electric power steering device, if steering is repeatedly executed, the electric current that flows through the motor or through a drive circuit that drives the motor brings about increases and decreases in the temperature of the motor or the drive circuit, and thus the motor or the drive circuit produces heat. Then, if the motor or the drive circuit overheats, there is a risk of occurrence of a trouble, such as a malfunction, a damaged component part, etc. In order to prevent the trouble caused by overheating, the related-art electric power steering device, for example, is provided with a temperature detector on a board where a control portion and the drive circuit are packaged, and thereby detects the temperature of the board. In the case where the temperature detected by the temperature detector exceeds a permissible temperature, the current supplied to the motor is restricted to or below a threshold value. Besides, the threshold value for restricting the current is increased or decreased according to the temperature of the motor or the drive circuit that corresponds to the history of steering. The threshold value is lowered when the temperature of the motor or the drive circuit becomes high, so that when the temperature of the circuit board becomes equal to or higher than the permissible temperature, a current restricted to or below the threshold value is supplied to the board or the motor. In consequence, further increases in temperature of various component parts on the board and of the motor are prevented, and therefore troubles caused by overheating are prevented.
In the electric power steering device that restricts current as described above, if the engine has been at rest for a long time, the temperature of the board disconnected from the supply of electric power sufficiently declines. Therefore, there is no need to execute a control of restricting current immediately after a starter switch for starting the engine is turned on. However, in the case where after the engine stops, the starter switch is turned on before the temperature of the board declines, there is a need to execute a control of restricting current immediately after the turning-on of the starter switch. Besides, generally in the electric power steering device, the board and the motor are provided apart from each other, so that there is a need to estimate the temperature of the motor from the temperature of the board.
Japanese Patent No. 3578048 proposes an electric power steering device in which, when the starter switch is turned off, the electric power supply to an electric power source unit that drives and controls the motor is discontinued, and then an electric power source is turned on, the temperature of the motor is estimated, and a control of restricting current is executed. The electric power steering device described in Japanese Patent No. 3578048 directly detects the temperature of a specific site in an electric control unit that drives and controls the motor, and estimates the temperature of the motor on the basis of a cooling time obtained from a detected temperature value, as well as cooling characteristics of the electric control unit and the motor. Then, on the basis of the estimated temperature of the motor, the control of restricting current to or below the set threshold value.
However, when the engine is at rest, the current to the steering-assist motor is discontinued, the temperature of the specific site in the electric control unit decreases to some extent during the off-state, whether the duration of the off-state is long or short. Therefore, in the above Japanese Patent No. 3578048 in which the temperature of the engine is estimated, and current is restricted on the basis of the estimated temperature, if an estimated temperature of the motor is lower than the actual temperature thereof, the current is less frequently restricted, and it can happen that although it becomes necessary to restrict the current, the current is not restricted, and the temperature of the motor or the electric control unit may possibly further increase. In consequence, there is a risk of occurrence of a trouble due to overheating. On the other hand, in the case where an estimated temperature of the motor is higher than the actual temperature thereof, there is a possibility of restricting the current when the current actually does not need to be restricted. Therefore, the electric power steering device of Japanese Patent No. 3578048 has a problem of the steering feel of a steering member changing dependently on the foregoing estimated temperature after the starter switch is turned on.